1. Field of the Invention
This invention relates to powder coating systems for spraying air entrained solid particulate powder material from powder spray guns onto workpieces within a powder spray booth. More particularly, this invention relates to an improved system for controlling and utilizing the finer particulate powder material in an electrostatic powder coating operation.
2. Description of the Prior Art
Powder spray systems have been used for many years for spraying air entrained solid particulate powder material from electrostatic powder spray guns onto workpieces within a powder coating booth. These systems conventionally collect oversprayed material and re-use it. The recycling system conventionally includes a fan for pulling the oversprayed powder coating material from the powder coating booth into a collector. The collector is comprised of a bank of cartridge filters which separates substantially all of the powder from the air and returns it to the spray guns, or a cyclone recovery system which removes principally only the medium and large powder particles from the air and returns those particles to the spray guns. In a cyclone system, finer particles which are not separated from the air in the cyclone are trapped in a bank of downstream filter cartridges or filter bags. These fines are normally scrapped.
A typical powder coating used in these systems has particles that range in size from submicron to 200 microns (.mu.). Systems for spraying and recycling powder coating materials, particularly those which use cartridge filter collectors as opposed to cyclone recovery systems, often times have problems associated with the buildup of excessive amounts of fine particles or "fines." Fines are generally defined as particles with a mean diameter of 10.mu. or less. Fines can also be defined in the relative sense as the smaller diameter particles in any given particulate size distribution for a sample of powder coating material.
There are various theories concerning why fine powder particles tend to build up in these systems. One theory is that, when the particles are sprayed from the gun towards the article to be coated, the heavier particles have greater momentum and, therefore, tend to reach the article or the area around the article more easily. The fine particles, on the other hand, have less momentum, and, therefore, have more difficulty breaking loose from the booth air currents. Another belief is that coarser particles are more easily charged than finer particles due to apparent greater attraction of charging ions to the larger particles. The greater charge on the larger particles causes them to be more strongly attracted to the part being coated. It is also believed that surface repulsion is a factor, that is, that even if the finer particle with its lower momentum, and lower charge, does reach proximity to the part, it is more likely to be repelled away from the part by the larger particles already on the part due to the fact that both the large and small particles are charged to the same polarity. Therefore, whereas a large charged particle is attracted to the electrical grounding surface presented by the article to be coated, the finer particle is more likely to be repelled by an article surface already covered by larger particles charged to the same polarity as the smaller particles.
The result of these factors, and possibly others, is that fine particles do not adhere to the part being coated with the same frequency as large particles and instead become entrained in the booth air flow leading into the collection system. In the case of a cyclone collection system many of these fines will not be separated from the air in the cyclone and will arrive at the final filtering system where they will be scrapped (i.e., wasted) and thereby removed from the system. In the case of a cartridge filter collection system the fines will be separated from the air with the other recovered powder. The fines returned from either the cyclone or cartridge recovery system will then be sprayed through the spray gun again where the same factors discussed above again come into play with the result that the fines most likely will again not adhere to the part and will be collected in the recovery system. The result will be, particularly with cartridge filter recovery systems, that the fine particles in the system will build up over time.
There are many deleterious effects of allowing finer particles to build up in a powder coating system. Too high a concentration of finer particles contributes to low film thickness, poor fluidization in the feed and reclaim hoppers, excessive agglomeration of fine particles (i.e., silting) in pumps, feed lines and guns, reduced efficiency of filters and sieves due to blinding of pores by the finer particles, and increased powder deposition on the guns and booth surfaces. One reason why finer particles are particularly attracted to the booth surfaces is the Van der Waals force which attracts small particles toward larger more massive particles such as the booth walls. Increased powder deposition on the booth walls makes the cleaning of the booth more difficult and time consuming when it is time to change from powder coating with one color of powder to another. Blinding of filters by the finer particles results in the more frequent replacement of filters which is inefficient, time consuming and costly.
To get the fines out of systems having cartridge filter recovery systems, some powder coating users will simply dump all the powder in the system when the fines concentration becomes excessive. In cyclone recovery systems the fines are more or less being continuously dumped, which reduces the fines concentration in the system. In either type of recovery system, however, the fines are often wasted and not effectively utilized.
To avoid having to periodically dump all powder from the system, or continually dump fines in cyclone systems, some powder coating users are now sending the fines back to the powder coating material manufacturer for reprocessing or forcing powder coating material manufactures to remove fines before shipping the powder. These methods, however, substantially increase the cost of the powder coating material to the users.